Utilizing measurement of several radio-labeled molecular probes of various sizes, alveolar epithelial solute permeability will be measured and expressed as an equivalent pore radius across the alveolar epithelium. One series of studies will focus on the response of alveolar epithelial permeability to changes in inflation of the lung in adult animals. The relationship between permeability of the alveolar epithelium to protein and the ability of the lung to maintain a liquid free alveolus will also be investigated. A second series of studies will define the relationships between the presence of pulmonary surfactants in fetal lung liquid and the changes in the solute permeability of the alveolar epithelium at the onset of ventilation. The perinatal experiments in immature fetal animals will also investigate the importance of alveolar epithelial protein permeability to the timing and completeness of fetal lung liquid absorption at the start of breathing.